FIG. 1 illustrates a typical power controller 10 that provides soft-startup and short-circuit protection for any load. The controller 10 receives an on/off control signal and, in response, turns pass transistor 12 on or off to couple the input voltage Vin at input terminal 14 to a load 16. The load may be a low-voltage electronic system.
The pass transistor 12 is an NMOS transistor whose gate voltage must be significantly above its source voltage in order to fully turn on. Since, ideally, the voltage applied to load 16 is approximately the same voltage as Vin, the gate voltage must be significantly higher than the input voltage Vin. Accordingly, a charge pump 18 is used to double or triple the input voltage, and the multiplied voltage is coupled to the gate of the pass transistor 12 in response to the on/off signal applied to terminal 20. Capacitively switched voltage doublers and voltage triplers are well known and need not be described. A gate filter capacitor 21 is commonly used to filter the signal from the charge pump 18.
The circuitry described above is commonly used to selectively apply power to various loads in a system, where a number of the circuits of FIG. 1 are connected to the same power supply. When the pass transistor 12 is turned on for a capacitive load (the load may include a large filter capacitor), there will be a large inrush current through the pass transistor 12. This inrush current is not only dangerous for the pass transistor 12 but it will momentarily lower Vin, causing a brown-out of other systems powered from the same supply line. To limit the current through the pass transistor 12, a current limiting circuit is employed. This current limiting circuit typically includes a low value current sense resistor R1, where the voltage across the resistor R1 is proportional to the current through the pass transistor 12. This voltage is applied to a differential amplifier 22 to obtain a voltage proportional to the current. The output of amplifier 22 is applied to an input of a second differential amplifier 24. The differential amplifier 24 has another of its inputs connected to a reference voltage that is set to a current threshold limit.
As the current through sense resistor R1 reaches the current limit, the output of differential amplifier 24 controls an NMOS transistor 26 to shunt current from the gate of the pass transistor 12 so as to limit the current through pass transistor 12 to at or below the threshold current. The input voltage Vin powers the differential amplifier 24 so that the maximum gate voltage to the NMOS transistor 26 is Vin.
The current limiting circuitry in FIG. 1 is also referred to as a hot-swap controller, since it allows the load 16 to be replaced with another load while avoiding the current surging that could cause a brown-out of other systems connected to the same supply voltage Vin.
In the event of a short circuit in load 16, it is desirable that the NMOS transistor 26 be quickly turned on to shut off pass transistor 12. The load may be low voltage electronic circuitry that can operate with voltages as low as 2 volts. With a low input voltage Vin, the turn on time of the NMOS transistor 26 is relatively high and the discharging capability of transistor 26 is relatively limited, delaying the turn off of the pass transistor 12. One solution for speeding up the turn-on time of the NMOS transistor 26 is to use a very large NMOS transistor 26. Another solution is to substitute transistor 26 with a bipolar transistor. Using a large NMOS transistor 26 undesirably uses up die area, and forming a bipolar transistor complicates the manufacturing process.
What is needed is a technique for quickly turning on NMOS transistor 26 that does not suffer the drawbacks described above.